The chronically catheterized fetal sheep has been utilized as an experimental model to study the endocrine effects, synthesis, and secretion of prolactin by the fetus. Plasma prolactin concentrations have been measured in individual animals over the critical period of augmented adrenal function occurring over the three to four weeks prior to parturition in this species. Perturbation experiments including infusion of exogenous prolactin, stimulation of endogenous prolactin with TRH, inhibition of endogenous prolactin with CB154, and varying combinations of surgical hypothalamic pituitary perturbations have been performed (hypophysectomy, stalk-section) in order to investigate the kinetics of prolactin production, clearance and adrenal placental steroidogenesis. Following experiments with exogenous infusion of ovine prolactin (NIH P-512), clearance rate of fetal prolactin was calculated at 2.65 plus or minus 0.53 (SEM) ml/minute or 1.51 plus or minus 0.3 (SEM) ml/minute/Kg. Fetal MCR of prolactin was related to fetal endogenous prolactin concentration. Following stimulation of endogenous prolactin with TRH, mean prolactin concentrations increase from 56.7 plus or minus 5.7 ng/ml to 130.7 plus or minus 5.4 ng/ml. TRH infused fetuses did not deliver prematurely, and fetal plasma cortisol concentration time trend effects were not significantly different from control animals. Elevation of prolactin did not therefore influence the onset of labor or glucocorticoid interrelationships. On inhibition of endogenous prolactin with CB154, mean fetal plasma prolactin fell from 105.0 plus or minus 16.4 ng/ml to 7.4 plus or minus 3.4 ng/ml. The CB154 infusion did not influence the duration of gestation. At this stage it appears that fetal prolactin does not play a major role in the effects on other major maturational processes remains unknown.